Nickel-iron battery with high cycle life

ABSTRACT

The present invention provides one with a high cycle life Ni—Fe battery. The battery uses a particular electrolyte. The resulting characteristics of cycle life, as well as power and charge retention, are much improved over conventional Ni—Fe batteries.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/761,312, filed Feb. 6, 2013, and U.S. Provisional ApplicationSer. No. 61/927,291, filed Jan. 14, 2014, which applications areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the technical field of energy storagedevices. More particularly, the present invention is in the technicalfield of rechargeable batteries using an iron anode and a nickelcathode.

2. State of the Art

Iron electrodes have been used in energy storage batteries and otherdevices for over one hundred years. Iron electrodes are often combinedwith a nickel base cathode to form a nickel-iron battery. Thenickel-iron battery (Ni—Fe battery) is a rechargeable battery having anickel (III) oxide-hydroxide cathode and an iron anode, with anelectrolyte such as potassium hydroxide. The active materials are heldin nickel-plated steel tubes or perforated pockets. It is a very robustbattery which is tolerant of abuse, (overcharge, overdischarge, andshort-circuiting) and can have a relatively long life. It is often usedin backup situations where it can be continuously charged and can lastfor more than 20 years. Due to its low specific energy, poor chargeretention, and high cost of manufacture, however, other types ofrechargeable batteries have displaced the nickel-iron battery in mostapplications.

The ability of these batteries to survive frequent cycling is due to thelow solubility of the reactants in the electrolyte. The formation ofmetallic iron during charge is slow because of the low solubility of theferrous hydroxide. While the slow formation of iron crystals preservesthe electrodes, it also limits the high rate performance. These cellscharge slowly, and are only able to discharge slowly. Nickel-iron cellsshould not be charged from a constant voltage supply since they can bedamaged by thermal runaway. The cell internal voltage drops as gassingbegins, raising the temperature, which increases current drawn and sofurther increases gassing and temperature.

The industry, however, would be greatly served by a nickel-iron batteryhaving improved performance. Such batteries, having even higher cyclelife, particularly in combination with improved charge retention,specific power and power density, would be greatly welcome. The uses ofnickel-iron batteries would thereby be increased.

SUMMARY OF THE INVENTION

The present invention provides one with a battery having an iron anode,e.g., a Ni—Fe battery, having improved performance characteristics. Thebattery uses a particular electrolyte. The resulting characteristics ofcycle life, charge retention, specific power and power density are muchimproved over such iron anode batteries in the prior art.

Among other factors, it has been discovered that when using an ironanode in a battery, the use of a particular electrolyte enhances theperformance characteristics of the battery significantly. Theelectrolyte is a sodium hydroxide based electrolyte. In one embodiment,a separator is used that is a non-treated polymeric separator, e.g.,made from a polyolefin. The result is a battery of enhanced power,capacity and efficiency. The cycle life can be improved tenfold comparedto the prior art.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is a perspective view of a coated iron anode;

FIG. 2 is a side view and cross-section view of an iron electrode coatedon both sides of the substrate;

FIG. 3 is a schematic of a battery in accordance with one embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

For the purpose of the present invention and description, the followingdefinitions will apply.

-   -   Capacity of a battery is measured in ampere hours (Ah).    -   Specific energy defines the battery capacity in weight, Watt        hours/kg (WH/kg). A battery can have a high specific energy but        poor specific power (load capacity), as is the case in alkaline        batteries. A battery may have a low specific energy but can        deliver high specific power, as is possible, e.g., with a        supercapacitor. Specific energy is often thought to be        synonymous with battery capacity and runtime.    -   Energy density, or volumetric energy density, is given in size,        Watt hours/liter (WH/L).    -   Specific power defines the battery capacity, or the amount of        current the battery can provide. Specific power is given in        Watts/kg (W/kg). Batteries for power tools, for example, often        exhibit high specific power but low capacity. Specific power        indicates internal resistance and the delivery of power.    -   Power density is the amount of power per unit volume. Power        density is given in Watts/liter (W/L).    -   C rate specifies charge and discharge currents. At 1 C, the        battery charges and discharges at a current that is par with the        marked Ah rating. At 0.5 C, the current is half, and at 0.1 C        the current is one tenth. For example, 1 C charges a battery in        about one hour; 0.5 C would take 2 hours and 0.1 C about 10        hours.    -   Watt hour efficiency is the energy discharged as a percentage of        energy charged.    -   Charge retention is the capacity measured after 28 days at 20°        C.    -   Cycle life of the battery is an important aspect, and is        measured at 80% DOD (depth of discharge), at 20° C., 1 C charge,        1 C discharge, to 70% capacity.

The invention comprises a battery with an iron anode and nickel cathode.The battery, in one embodiment comprises an iron electrode comprised ofa single, coated conductive substrate, prepared by a simple coatingprocess, which can be continuous. The substrate can be coated on oneside, or on both sides.

The battery is prepared by conventional processing and construction withan iron anode and a cathode, i.e., a nickel cathode. The battery of thepresent invention, however, comprises a particular electrolyte. In oneembodiment, the nickel-iron battery also comprises a particularseparator, e.g., comprised of a polyolefin. In one embodiment, the ironelectrode is comprised of a single, coated conductive substrate, asshown in FIGS. 1 and 2.

Turning to the figures of the drawing, FIG. 1 is a prospective view of acoated iron electrode. The substrate 1 is coated on each side with thecoating 2 comprising the iron active material and binder. This isfurther shown in FIG. 2. The substrate 10 is coated on each side withthe coating 11 of the iron active material and binder.

The electrolyte used is a sodium hydroxide based electrolyte, with thesodium hydroxide generally having a concentration of 5-7N in theelectrolyte. In one embodiment, the electrolyte comprises sodiumhydroxide, lithium hydroxide and sodium sulfide. For example, the sodiumhydroxide concentration in the electrolyte is about 6N, the lithiumhydroxide concentration in the electrolyte is about 1N, and the sodiumsulfide concentration in the electrolyte is about 2 wt %. In using thiselectrolyte with an iron anode battery, it has been discovered that thelife, capacity and power of the battery is much improved. It is believedthat the use of the sodium hydroxide based electrolyte reduces the ironsolubility in the electrolyte, which extends the battery life. Theentire cell is also more stable and effective at high temperatures. Thelithium hydroxide increases charge acceptance of the positive electrode,which increases capacity.

The presence of a metal sulfide has been discovered to be important forthe effective deposit of sulfur on the iron anode. A battery with aniron anode seems to work better with sodium sulfide in the electrolyte,as the sulfide ends up in the iron anode as a performance enhancer aftera few cycles. The sodium sulfide in essence is believed to increase theeffective surface area of the iron, so one obtains more utilization ofthe iron. The capacity and power is therefore improved. In addition, theadded sulfide is believed to form iron sulfides, two of the forms beingFeS and Fe₂S₃, both of which are more electrically conductive thanFe(OH)₂ which normally forms on the iron surface. These conductive siteson the iron surface create a situation in which the oxidized layer thatforms on the iron surface is thicker before true electrical passivationoccurs allowing for more efficient use of the underlying iron activematerial. Various sulfide salts may be employed to obtain this desirableresult. In one embodiment, the sulfide salt is sodium sulfide. Overall,it has been found that use of the present electrolyte improves cyclelife, as well as the power and the capacity (charge retention) of astandard Ni—Fe battery.

While the use of metal sulfides such as sodium sulfide is describedabove, it will be understood that other sulfide compounds of suitablesolubility may also be used. Examples of such sulfides include inorganicsulfides with sufficient solubility, but also organic sulfur compoundsknown to decompose in the electrolyte to inorganic sulfide.

It has also been found that the concentration of sulfide per se in theelectrolyte can be important. In one embodiment, the amount of sulfideper se, i.e., the amount of sulfide itself, as measured as a percentageof the weight of electrolyte, is from 0.23% to 0.75%. In one embodiment,the amount of sulfide per se, measured as a percentage of the iron inthe electrode, ranges from 0.23 wt % to 0.75 wt %.

The metal sulfide is preferably Na₂S. The sodium sulfide can be, forexample, hydrated Na₂S. Hydrated sodium sulfide is about 60% Na₂S byweight, and this must be considered in calculating the amount of sulfideper se used in the electrolyte. In general, the amount of Na₂S used inthe electrolyte ranges from 1-2 wt %, based on the weight of theelectrolyte.

In one embodiment, the concentration of the NaOH in the electrolyte isin the range of from 6 to 7.5M. In one embodiment, the amount of LiOH inthe electrolyte is in the range of from 0.5 to 2.0M, and most preferablyabout 1.0M. The combination of NaOH with LiOH and a sulfide is unique inits effective result.

It has also been discovered that using the electrolyte described abovein combination with an iron electrode coated onto a single substratesignificantly reduces the time required for activation of the cell orbattery. In particular, use of this electrolyte in conjunction with anadhering type of iron electrode comprising iron active materials pastedonto a conductive substrate such as a metal foil or foam, results in abattery with improved performance over Ni—Fe batteries of conventionalpocket plate design. Performance is further improved if such an adheringtype of iron electrode contains sulfur or sulfide additives.

The battery separator that can be used in the present battery incombination with the electrolyte, is one that is iron-phobic. Theseparator can be etched for wettability, but this is merely optionalwhen using the present battery separator. The battery separator is madeof a polymer, with a generally smooth surface. The polymer can be anypolymer which provides a non-polar surface, which is also generally verysmooth. Examples of such polymers include polyolefins, such aspolyethylene, and polytetrafluoroethylene (e.g., Teflon®). By using aseparator which is more iron-phobic, the separator picks up iron at aslower rate. This results in a significant increase in the cycle life ofthe battery. Use of the e.g., polyolefin, separator in combination withsodium hydroxide electrolyte has been discovered to improve thecapacity, power and, the efficiency, but most importantly, the cyclelife of a standard Ni—Fe battery at least threefold.

FIG. 3 depicts a battery 20 with an iron anode 21. A nickel cathode 22is also in the battery. The electrolyte 23 surrounds both the iron anodeand cathode. The electrolyte is the sodium hydroxide based electrolytedescribed above, comprising sodium hydroxide, lithium hydroxide andsodium sulfide. The battery separator 24 is in one embodiment aniron-phobic battery separator having a non-polar surface. The batteryseparator can be made of any substance that provides such a non-polarsurface. Polymers are good candidates as they provide smooth andnon-polar surfaces. Suitable polymers include the polyolefins.

The battery can be made using conventional means and processes. However,the anode must be an iron anode, and an electrolyte comprising sodiumhydroxide, lithium hydroxide and sodium sulfide is used. In oneembodiment, both the sodium hydroxide based electrolyte and aniron-phobic battery separator are used in the battery. A great benefitof using the three component sodium hydroxide based electrolyte is thatthe battery can be sealed. A typical flooded design need not be used.Such a sealed battery is maintenance free as electrolyte need not beadded periodically, as one would with a flooded design.

In one embodiment, the iron anode itself is different from thetraditional pocket anode design. The anode is a single, coatedconductive substrate, which can be coated on one side, or both sides.The anode can also be made by a simple coating process, which can becontinuous.

The single substrate of the iron anode is used as a current conductingand collecting material that houses the active material (iron) of theelectrode. In the traditional pocket design, the substrate encompassesthe active material and holds the material. Two layers of substrate aretherefore required per electrode. In the single substrate design, asingle layer of substrate is used. This single layer acts as a carrierwith coated material bonded to at least one side. In one embodiment,both sides of the substrate are coated. This substrate may be a thinconductive material such as a metal foil or sheet, metal foam, metalmesh, woven metal, or expanded metal. For example, a 0.060 inch, 80 ppi,nickel foam material has been used.

The coating mix for the iron anode is a combination of binder and activematerials in an aqueous or organic solution. The mix can also containother additives such as pore formers. Pore formers are often used toinsure sufficient H₂ movement in the electrode. Without sufficient H₂diffusion, the capacity of the battery will be adversely affected. Thebinder materials have properties that provide adhesion and bondingbetween the active material particles, both to themselves and to thesubstrate current carrier. The binder is generally resistant todegradation due to aging, temperature, and caustic environment. Thebinder can comprise polymers, alcohols, rubbers, and other materials,such as an advanced latex formulation that has been proven effective. Apolyvinyl alcohol binder is used in one embodiment.

The active material for the mix formulation of the iron anode isselected from iron species that are generally less oxidative. Suchmaterials include metal Fe and iron oxide materials. The iron oxidematerial will convert to iron metal when a charge is applied. A suitableiron oxide material includes Fe₃O₄. In addition, any other additives aregenerally required to be of a less oxidative nature, such as sulfur,antimony, selenium, and tellurium.

The coating method can be a continuous process that applies the activematerial mixture to the substrate by a method such as spraying, dip andwipe, extrusion, low pressure coating die, or surface transfer. A batchprocess can also be used, but a continuous process is advantageousregarding cost and processing. The coating mixture has to maintain ahigh consistency for weight and thickness and coating uniformity. Thismethod is conducive to layering of various materials and providinglayers of different properties such as porosities, densities andthicknesses. For example, the substrate can be coated with three layers.The first layer being of high density, second layer of medium density,and final layer of a lower density to create a density gradient whichimproves the flow of gases from the active material to the electrolyte,and provides better electrolyte contact and ionic diffusion with theactive material throughout the structure of the electrode.

After coating, the electrode is dried to remove any residual liquid,i.e., aqueous or organic solvent. The drying methods will generallyprovide a continuous method for liquid removal from the coated activematerial which will enhance the adhesion and binding effects of the dryconstituents without iron ignition. This drying method provides auniform and stable active material coating with the substrate material.Two stages of drying can be used. For example, the first can beradiation for bulk drying, for cost and quality control, followed byconvection drying to remove the remaining liquid. The radiation used canbe any radiation, such as infrared, microwave or UV, and is very fast.However, the radiation creates a high temperature at the surface of thecoated electrode. The high temperature is fine as long as water is stillpresent to act as a heat sink. Therefore, the water is generally removedto about 10-20 wt % water. This can generally be determined using acontrol chart. Going below 10% water is dangerous, as the electrodebecomes too dry and the high temperature can ignite the iron. Thus,using the convention drying to complete the removal of water/liquid is apreferred embodiment, once the amount of water remaining is in the 10-20wt % range. In another embodiment, radiation can be used to complete thedrying if the process is conducted in an inert atmosphere.

The compaction methods used can be accomplished by rolling mill,vertical pressing, and magnetic compaction of the active material to thedesired thickness from 0.005 to 0.500 inches and porosities from 10% to50%, for high quality and low cost continuous processing. In oneembodiment, the porosity of the electrode is from 15-25% porosity. Thiscompaction method can be used in conjunction with the layering methoddescribed above for providing material properties of density, thickness,porosity, and mechanical adhesion.

In addition, continuous in-line surface treatments can be appliedcontinuously throughout any of the steps including coating, layering,and drying processes. The treatments can apply sulfur, polymer, metalspray, surface lament, etc.

The present batteries including the iron electrode can be used, forexample, in a cellphone, thereby requiring an electrode with only asingle side coated. However, both sides are preferably coated allowingthe battery to be used in numerous additional applications.

The resulting battery shows improved performance characteristics, and inparticular an improved cycle life, e.g., a cycle life of at least 10,000cycles, when cycled @ 80% DOD, at 20° C., 70% capacity. Comparison to astandard Ni—Fe battery of flooded design, the present high cycle Ni—Febattery has been found to exhibit the following characteristics:

Invention Conventional WH/kg (specific energy) 105 50 WH/L (energydensity) 183 40 W/kg (specific power) 2,100 100 W/L (power density)3,660 80 WH efficiency 95% 65% Charge Retention 95% 60% (Capacity @ 28days 20° C.) Cycle Life (@80% DOD, 20° C., 10,000 1,000 1 C Charge, 1 CDischarge, to 70% Capacity) In the foregoing table, WH is Watt hours.

In the foregoing comparison, the invention Ni—Fe battery used anelectrolyte comprised of sodium hydroxide (NaOH), lithium hydroxide(LiOH), and sodium sulfide (Na₂S). The separator used in the inventiveNi—Fe battery was a 0.010 inch thick polyolefin non-woven mesh. Theelectrolyte used in the conventional Ni—Fe battery was potassiumhydroxide (KOH), and the battery separator was 0.060 inch thickpolyvinyl chloride (PVC) windows. The results show a vast improvement inperformance characteristics for the inventive Ni—Fe battery. The Ni—Febattery, in addition to having a cycle life of at least 10,000 cycles,can also have a capacity at 28 day, 20° C., of at least 95%; a powerdensity of at least 3,660 W/L and specific power of at least 2100 W/kg.

While the foregoing written description of the invention enables one ofordinary skill to make and use what is considered presently to be thebest mode thereof, those of ordinary skill will understand andappreciate the existence of variations, combinations, and equivalents ofthe specific embodiment, method, and examples herein. The inventionshould therefore not be limited by the above described embodiment,method, and examples, but by all embodiments and methods within thescope and spirit of the invention.

What is claimed is:
 1. A battery comprising a nickel cathode, an ironanode with the battery exhibiting a cycle life of at least about 10,000cycles.
 2. The battery of claim 1, further comprising an electrolytecomprised of sodium hydroxide, lithium hydroxide and a sulfide.
 3. Thebattery of claim 1, further comprising a polyolefin battery separator.4. The battery of claim 1, which is a sealed battery.
 5. The battery ofclaim 1, wherein the iron anode is comprised of a single layer of aconductive substrate coated on at least one side with a coatingcomprising an iron active material and a binder.
 6. The battery of claim5, wherein the substrate is coated on both sides.
 7. The battery ofclaim 1, further exhibiting a specific energy of at least about 105 watthours/kg.
 8. The battery of claim 1, further exhibiting an energydensity of at least about 183 watt hours/liter.
 9. The battery of claim1, further exhibiting a specific power of at least about 2100 watts/kg.10. The battery of claim 1, further exhibiting a power density of atleast about 3660 watts/liter.
 11. The battery of claim 1, furtherexhibiting a watt hour efficiency of at least about 95%.
 12. The batteryof claim 1, further exhibiting a charge retention, measured as capacityat 28 days 20° C., of at least about 95%.
 13. The battery of claim 1,exhibiting a specific power of at least about 2100 watts/kg; and a powerdensity of at least about 3660 watts/liter.
 14. The battery of claim 13,exhibiting a specific energy of at least about 105 watt hours/kg; anenergy density of at least about 183 watt hours/liter; a watt hourefficiency of at least about 95%; and a charge retention of at leastabout 95%.
 15. The battery of claim 2, exhibiting a specific power of atleast about 2100 watts/kg; and a power density of at least about 3660watts/liter.
 16. The battery of claim 15, exhibiting a specific energyof at least about 105 watt hours/kg; an energy density of at least about183 watt hours/liter; a watt hour efficiency of at least about 95%; anda charge retention of at least about 95%.